So what does the constitution actually say? Here is the so-called Postal Clause:

(The Congress shall have power) ... to establish Post Offices and post Roads;

Of course, to adherents of original intent, this can only mean dirt roads for walkers, horses, and wagons--since cars and asphalt hadn't been invented yet.

The postal clause is cited as the federal government's authority to build nationwide highways, especially the interstate system. Of course, the basic interstate system being long complete, the money the feds pour into highways now is really for local transportation. The 80 may be national infrastructure, but most of the traffic on the 280 are commuters--for that matter, so is most of the traffic on the 80, from about Roseville on west.

The usual progressive pro-transit argument that follows this point is that, if the federal government supports local car travel, it should support local transit as well.

It seems fair, but is the best answer? Is routing transportation funding through Washington optimal? Hardly. It motivates regions to fight to "get their share" of money, instead of solving problems in ways that are sustainable. And I'm pretty certain that in the current arrangement, those of us who live in older metro regions (which tend to be disproportionately donors to the federal budget) taking transit, or at least driving relatively shorter distances, are actively subsidizing new freeways out in the boonies, paying a bunch of freeloaders to drive their monster-truck SUV's across vast swaths of exurbia...

If regions had to pay for their own transportation, we would probably see more effort to coordinate transit and development, so that their investment (of their own money) would show a better return. Proposals to dig tunnels anywhere some NIMBY objected to hearing to trains would probably receive the laughing dismissal they deserve. Some cities might even figure out they could solve their "transportation problems" without spending money on infrastructure at all, just by making it convenient for people to live close to where they work.

Of course there are reasons to keep the federal government involved in local transportation--one of the more important practical ones being that it's the part of the government that we pay most of our taxes to, so that's where the money is. There are plenty more. (This article was inspired as an answer to a smug, overly simplistic view of the constitution. I don't want to oversimplify on my part--I'll freely admit, transportation funding is messy and complicated, and probably has to be). But given the state of the national budget, local funding for transportation is becoming more and more of a reality anyway, and transit advocates should look for ways to turn this to their advantage.